This invention relates to lubricating materials.
Wear, defined as the removal of material from a solid surface, may be categorized as adhesive, abrasive, corrosive, surface fracture, erosive and fretting. Adhesive wear is thought to be most common, followed by abrasive wear.
Adhesive wear originates in the attractive atomic forces as two surfaces are brought together. Adhesion may result in material removal from either surface, depending upon material and environmental factors. Quantitatively, for adhesive wear, wear is directly proportional to load and sliding distance, and inversely proportional to the worn surface hardness.
Abrasive wear occurs when a hard surface slides across a softer surface creating grooves with a loss of material. Abrasive wear can be classified as two or three-body wear. Two-body occurs when a hard, rough surface interacts with another surface, while three-body wear occurs when hard, abrasive particles interact at the sliding interface of two other materials.
Lubricants are used to prevent contact of parts in relative motion and thereby reduce friction and wear. The most effective method of lubrication is to provide a hydrodynamic film between two surfaces, in which a fluid film is drawn into the contact area between two sliding surfaces. The coefficient of friction can be very low and with good design and maintenance, component lives can be very long.
Under high load conditions, the lubrication regime is elastohydrodynamic, in which lubrication is influenced by the elastic properties of the substrates. Under very high load, surface asperities come into contact and boundary lubrication begins. Boundary lubrication occurs frequently in high load sliding applications. For example, the hypoids used in automotive rear-axle transmissions operate under such severe conditions of load and sliding speed, with resulting high temperature and pressure, that ordinary lubricants cannot provide complete protection against metal contact. For such applications, extreme pressure additives are employed which provide a source of renewable surface boundary lubricant. Such additives react at hot spots on the surface to form a solid lubricating surface.
For most lubrication applications, fluids are adequate and perform remarkably well especially when formulated with additives, e.g., anti-oxidants, dispersants, and/or viscosity improvers to enable their use over a wide temperature range. However, applications under extreme environmental conditions, such as very high temperature or vacuum, preclude the use of fluid lubricants. Under such conditions, a solid lubricant may be employed in place of a liquid lubricant.
Solid lubricants are any solid material which may be used between two surfaces to provide protection from damage during relative movement to reduce friction and wear. Some advantages of solid lubricants include good stability at extreme temperatures and in chemically active environments, high load lubricating capacity and light weight. Some disadvantages include higher coefficient of friction as compared to hydrodynamic lubrication, solid sliding contact wear, finite lifetime and lack of cooling capability.
Two of the most useful solid lubricants are graphite and molybdenum disulfide. Both of these materials lubricate by shearing interlaminarly with small forces while carrying high normal loads. It has been observed that adsorbed water and gases improve the lubrication performance of graphite, while such adsorbed species are not required for molybdenum disulfide lubrication. Therefore, at higher temperatures, oxidatively stable graphite loses adsorbed species and, thus, much of its lubricating ability. In contrast, molybdenum disulfide lubricates effectively to relatively high temperatures without such adsorbed species, but oxidizes at lower temperatures than graphite.
Antimony trioxide is known to enhance the tribological performance of molybdenum disulfide in air from ambient temperature to about 315.degree. C. MoS.sub.2 begins to oxidize to MoO.sub.3 at about 315.degree. C. Between about 315.degree. C. and 370.degree. C., oxidation is rapid. It has long been thought that oxidation of MoS.sub.2 to MoO.sub.3 is to be avoided. Various hypotheses have been offered for the beneficial effect of Sb.sub.2 O.sub.3 addition to MoS.sub.2, that Sb.sub.2 O.sub.3 oxidizes sacrificially to retard the oxidation of MoS.sub.2, that Sb.sub.2 O.sub.3 forms an unharmful or even beneficial eutectic with MoO.sub.3, or that Sb.sub.2 O.sub.3 has some other undefined antioxidant role. The consensus has been that any quantity of MoO.sub.3 in MoS.sub.2 is to be avoided, if possible.
Recent work indicates some change in attitude; that MoO.sub.3 may not be detrimental to the lubricating property of MoS.sub.2. Hitachi, Japanese Pat. No. 0215494 disclose that a solid lubricating material made of MoS.sub.2 has oxidation temperature increased by the addition of 0.1 to 10 weight percent of an acidic substance selected from the group consisting of PbO, VO.sub.2, PbS, MoO.sub.3, BiO.sub.2, Mn oxide, Cu fluoride, W, B, P, Sb, Bi, Co or C. Fleischauer, P.D. and Bauer, R, "The Influence of Surface Chemistry on MoS.sub.2, Transfer Film Formation", ASLE Preprint No. 86-AM-5G-Z, observe that rf sputtered films of MoS.sub.2 whose surface layers have from 30 to 40 percent oxidized molybdenum have the longest wear lifetimes. The authors propose that oxidation of some portion of the MoS.sub.2 in the interface region between the film and the substrate results in longer wear lifetimes because such oxidation promotes good adhesion of the transfer lubricant film. They envision a graded interface that consists of a transition from metal to mixed oxides and sulfides to nominally pure MoS.sub.2, with the mixed region being only a few atomic layers thick.
I have discovered that the bulk addition of MoO.sub.3 to MoS.sub.2 in amounts of about 15 to 50 mole percent MoO.sub.3 improves the tribological performance of MoS.sub.2.
Accordingly, it is an object of the present invention to provide an improved solid lubricant.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art.